Ocean Surface a Boon for Extreme Event Forecasts, Warnings

Satellites passed over the Indian Ocean tsunami of December 2004. Two of those satellites--Jason 1 and Topex/Poseidon--were equipped with altimeters that for the first time measured the height of a tsunami in the open ocean.Credit: NASA/JPL

For humans in the path of destructive hurricanes and tsunamis, an accurate warning of the pending event is critical for damage control and survival. Such warnings, however, require a solid base of scientific observations, and a new satellite is ready for the job.

The Ocean Surface Topography Mission (OSTM)/Jason 2 adds to the number of eyes in the sky measuring sea surface and wave heights across Earth's oceans. The increased coverage will help researchers improve current models for practical use in predicting hurricane intensity, while providing valuable data that can be used to improve tsunami warning models.

"When it comes to predicting hurricane intensity, the curve in the last 40 years has been somewhat flat, with little advance in how to reduce error in predicted intensity," said Gustavo Goni, of the National Oceanic and Atmospheric Administration (NOAA) in Miami. Maps of sea surface height created from satellites, however, could help change the curve.

Satellites that measure sea surface height have been running operationally nonstop since November 1992. But more than one is needed to fly at the same time in order to identify all the features that could be responsible for intensification of tropical cyclones all over Earth. The OSTM/Jason 2 mission will help make the additional coverage possible.

NASA, university and NOAA investigators, including Goni, work to transform sea surface height information obtained from satellites, such as OSTM/Jason 2, into maps of ocean heat content. Forecasters can use the maps to develop models to predict how hurricanes will strengthen.

Determining heat content from sea surface height is possible because warm water is less dense and hence sits higher than cooler water. In some regions, such as inside and outside the Gulf Stream current, the temperature differences result in more than a one-meter (three-foot) difference in sea surface height. Goni and colleagues use this established concept to estimate from sea level variations how much heat is stored in the upper ocean in areas where hurricanes typically develop and intensify.

While sea surface height may not necessarily be the most significant parameter for hurricane intensity forecasts, researchers now know that if sea surface height is accounted for in current forecast models, errors in forecasts for the most intense storms are reduced. For weak storms, the reduction in error is not very significant. However, for storms in the strongest category 5 range, the heat content in the upper ocean derived from sea surface height becomes increasingly important. "This is a good thing, because these are the storms that produce the most damage," Goni said.

"OSTM/Jason 2 will help us to keep the necessary coverage that we need to identify ocean features that can be linked to tropical cyclone intensification, because with only one satellite we may miss some of them," Goni said.

Upper ocean heat content derived from sea surface height is now used in operational and experimental forecast models in all seven ocean basins where tropical cyclones exist.

In December 2004, two satellites happened to be in the right place at the right time, capturing the first space-based look at a major tsunami in the open ocean. Within two hours of a magnitude 9 earthquake in the Indian Ocean southwest of Sumatra, the Jason 1 and Topex/Poseidon satellites fortuitously passed over the path of the resulting tsunami as it traveled across the ocean. It measured the leading wave, traveling hundreds of miles per hour in the open ocean, at about 0.5 meters (1.6 feet) tall.

Wave height measurements like those of the Indian Ocean tsunami do not provide an early warning because the information is not relayed to ground stations in real time. That's the job of early warning systems operated by NOAA and other global organizations that currently employ a network of open-ocean buoys and coastal tide gauges. Sea surface height measurements of tsunamis can, however, help scientists test and improve ground-based models used for early warning. One such system developed at NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., and undergoing tests at NOAA's Pacific Tsunami Warning Center, Ewa Beach, Hawaii, could become operational within about three years.

Most tsunamis are caused by undersea earthquakes. Using the JPL-developed system, when seismometers first identify and locate a large earthquake, scientists can use GPS measurements to search around the earthquake's source to see if land has shifted, potentially spurring a tsunami. Scientists can then immediately compile the earthquake's size, location, and land movement into a computer program that generates a model tsunami to determine the risk of a dangerous wave. After the wave passes, scientists can search through wave height data from satellites and verify what the model predicted.

"Satellite data play the crucial role of verifying tsunami models by testing real tsunami events," said JPL research scientist Tony Song. "If an earthquake generates a tsunami, does the satellite data match observations on the ground and model predictions?"

"One of the unique pieces of satellite observations is the large-scale perspective," said JPL research scientist Philip Callahan. Tsunamis can have waves more than 161 kilometers (100 miles) long. Such a wave would likely go unnoticed by an observer in a boat on the ocean's surface. But satellite altimeters like OSTM/Jason 2 can see this very long wave and measure its height to an accuracy of about 2.5 centimeters (one inch).

Scientists' ability to test tsunami warning models will be aided by OSTM/Jason 2. With the Topex/Poseidon mission now ended, the currently orbiting Jason 1 has now been joined by and will eventually be replaced by OSTM/Jason 2. This will help ensure that future tsunamis will also be observed by satellites as well as by buoys and tide gauges.

"The biggest value in satellite measurements of sea surface height is not in direct warning capability, but in improving models so when an earthquake is detected, you can make reliable predictions and reduce damage to property and people," Callahan said.